Characteristics of Crustacea. cxxi 



tracheal system, but which with more probability have been con- 

 sidered to have a glandular function, and to keep the gill-plates 

 lubricated and transpirable by their secretion. 



The nerve-system resembles that of other Arthropoda in its ge- 

 neral arrangement^ but differs in the two following points : — The 

 commissural cords connecting their prae-oral and post-oral ganglia 

 are of so much greater length, that their 'brain' is never described 

 as that of other Arthropoda sometimes is, as consisting of an infra- 

 oesophageal as well as of a supra-oesophageal mass, the ganglia of 

 the jaw-bearing segments having receded so as to become apposed 

 to the first of the thoracic series, with one or all of which they are 

 always fused, whilst the ganglia of the two antennary segments 

 maintain their usual continuity with those of the eye-bearing seg- 

 ment. And, secondly, the paired^ and the azygos systems of stomato- 

 gastric nerves, except in the terrestrial Isopoda, take origin from 

 the commissural cords of the nerve collar, and from the prae-oral 

 ganglionic mass instead of from any distinct ganglia, such as the 

 lateral and frontal ganglia of other Arthropoda. The nervi transversi, 

 however, appear to be represented in Crustacea by a nervous stem, 

 passing off from each interganglionic segment of the cord, as is the 

 ease in other Arthropoda, and indeed also in those Vermes which 

 possess a chain of ventral ganglia. The eyes may vary from the very 

 simplest up to the most complex form observed in the Sub-kingdom ; 

 two kinds may co-exist in the same individual, and in two genera, 

 Etqihaiisia and Thysanopoda, eyes may be, contrary to the otherwise 

 invariable rule in Arthropoda, found elsewhere than upon the head. 

 An auditory organ has been observed in Decapoda in the basal joint 

 of the superior antennary organs; and in IIt/sis, in the lateral 

 appendages of the last post-abdominal segment, which, together 

 with the two pairs of antennary organs, were the only appendages 

 developed in the earliest period of its larval life. 



With the exception of the Cirripedia, all the Crustacea are dioe- 

 cious. 'Complementary males' are found in the order Cirripedia, 

 and also amongst the parasitic Entomostraca, and in the former 

 order these 'pygmy' males are devoid of any digestive tract, and 

 even of any oral opening. The sessile-eyed Crustacea furnish an 

 exception to the rule, that in Arthropoda the females are larger in 

 size than the males. The generative organs have bilaterally sym- 

 metrical ducts, opening a considerable distance anteriorly to the 



